As the person skilled in the art is aware, to each type of transport network there corresponds at least one type of data transport frame. Thus in PDH-type synchronous networks data is sent in the form of E1-type frames and in similar frames (in particular T1, J1, E2, T3 and J3 frames) whereas in asynchronous networks data may be sent in the form of Ethernet frames.
For a network of a second type (for example Ethernet) to be able to transport frames coming from a network of a first type (PDH) transparently, without losing information and if possible with a low and guaranteed transmission delay, the connection interfaces of the networks must include equipment capable of converting (for example encapsulating) at least some of the frames that they receive.
Various solutions for effecting such conversion have been proposed.
The PICMG standardization body has proposed a solution known as I-TDM consisting in transporting over Ethernet TDM (time division multiplex) channels containing some of the data contained in the E1-type frames. A drawback of that technique is that the TDM data bytes are extracted from the E1 frame by E1 frame builders (also known as E1 framers), which is less than the optimum for implementing certain functions offered by certain recent architectures known as frame termination architectures, such as the aTCA architecture, because of the presence in the Ethernet frame of a header for each TDM channel (also known as the overhead).
An aTCA architecture offers telecommunications equipment modularity but does not offer independence of E1-type line protection (automatic protection switching (APS)) and line termination board protection (equipment protection switching (EPS)).
The IETF standardization body, and more particularly its working group PWE3, has proposed a CES (circuit emulation services) solution and a solution for transporting TDM channels over the Internet Protocol known as TDMoIP (TDM over IP). Those two solutions consist in Internet Protocol (IP) transport protocols whose respective transport layers generate large headers and therefore limit the transport capacity of the network, i.e. the number of E1-type links (or lines) that can be transported by a network of the Giga Ethernet type.